The present invention relates to making composite structures for electronics, optics or microelectronics.
More precisely, the invention relates to a method of detaching two substrates at an embrittlement zone situated at a given depth of one of the two substrates, the method comprising a separation annealing step implemented in a furnace, said annealing comprising                a first phase during which the temperature changes along an upgrade allowing a high temperature to be reached and stabilizing the annealing at this high temperature,        a second phase during which the temperature changes along a downgrade, at the end of which the furnace is opened and then the substrates are unloaded from the furnace.        
The invention also relates to a Silicon On Insulator (SOI) type structure obtained by such a method.
Applying such methods to carry out detachment between two substrates, between which an embrittlement surface was defined by implantation, is known.
For example, thermal separation annealing is thus one of the steps in the Smart Cut™ method used to produce SOI type composite structures.
During separation annealing, the SOI surface (buried oxide layer and superficial silicon layer) is transferred from a donor substrate (previously oxidized and implanted) to a receiver substrate (which was bonded to the donor substrate).
At the end of the separation annealing, the donor substrate is detached from the receiver substrate.
Thus, an SOI type structure is obtained, that may also be designated as a “positive substrate” and a “residual substrate;” the residual substrate may be designated as a “negative substrate,” and may be recycled.
It is specified that the separation annealing is implemented in a furnace.
After separation annealing, a defect known as a “cleavage line” is sometimes observed.
This defect may be characterized as follows:                the defect appears near the center of the negative substrate;        the defect may cross through the entire thickness of this negative substrate;        the defect propagates by moving along the crystallographic directions of the crystal constituting the negative substrate;        the defect may propagate until a sufficient size to cause breakage of the negative substrate is reached.        
Such a “sufficient size” corresponds to the case where the defect propagates on the entire diameter of the substrate and the substrate then “spontaneously” breaks or to the case where the defect sufficiently weakens the substrate so that the substrate breaks during possible mechanical stress.
The appearance of a “cleavage line” defect on the negative substrate does not allow the substrate to be recycled, which corresponds to a loss.
In addition, because the negative substrate is in contact with the positive substrate during separation, the “cleavage line” defect may damage the positive substrate.
In fact, as the two substrates are in close contact, the defect on the negative substrate may lead to the positive substrate being damaged.
This is harmful, since the positive substrate will provide the final SOI structure after annealing.
FIGS. 1a and 1b present maps of defects present on two SOI structures (positive substrate) against which the negative substrate had presented a “cleavage line” defect.
These maps are obtained by using a piece of KLA Tencor SP2 type inspection equipment allowing the final defectivity of the SOI structure to be measured.
In the case of FIG. 1a, the negative substrate presents a complete “cleavage line” defect that led to breakage of the negative substrate. The negative substrate defect is definitely propagated on the SOI structure.
In the case of FIG. 1b, the negative substrate presents the beginning of a “cleavage line” present in the center. In the same way as the case illustrated in FIG. 1a, the defect is also propagated on the SOI structure.